(1) Field of the Invention
The present invention relates to maintenance of a constant linear relative speed between a disc on a turntable and the stylus of a pick-up device which is in contact with the surface of such disc. More specifically, this invention is directed to control apparatus for maintaining the linear speed of a turntable relative to radially movable member constant. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
(2) Description of the Prior Art
A conventional phonograph disc recording is produced on a recording lathe rotating at a constant angular rate while a cutting head is moved radially along a horizontal overhead track. The number of grooves per inch to be inscribed upon the record surface is dependent upon the speed with which the cutting head is moving toward the center of the record. This speed is not constant because the groove spacing must vary according to the peak to peak amplitude of the signal to be cut on the record, which is adjusted automatically on contemporary recording lathes, and to provide lead-in and lead-out grooves and intersectional spaces between the various selections on a side of the record. Thus, the groove as inscribed upon the recording surface will take the form of an extremely irregular spiral about the center point of the record from a radius of about 5.75 inches to 2.00 inches on a conventional 12 inch diameter record with a standard 4 inch diameter label.
The linear speed of the record, i.e., the rate of travel of the recording surface beneath the recording or reproducing stylus; as the record rotates at constant angular speed will vary in proportion to the instantaneous radial position of the stylus. As the stylus follows the groove spiral inward, the linear speed will vary by a factor of 2.88, which is the outermost radius (5.75) divided by the innermost radius (2.00). Table I gives the actual linear speed at various radii of a conventional 33.33 R.P.M. long-playing record.
TABLE I ______________________________________ Radius (inches) Linear Speed (inches/second) ______________________________________ 5.75 20.00 5.00 17.40 4.87 17.00 4.00 14.00 3.00 10.50 2.50 8.70 2.00 6.96 ______________________________________
The linear speed of the record determines the highest usable frequency that can be recorded. This maximum frequency occurs when the wavelength of the signal inscribed on the record (wavelength in this case being the linear speed in inches per second divided by the frequency of the modulating signal in cycles per second) becomes comparable to the dimensions of the cutting or reproducing stylus. This can be influenced by the stylus shape, stylus contact area, groove shape, hardness and elasticity of the record surface, etc. Using a contemporary standard commercial long-playing record as a basis, it is empirically true that a linear speed of approximately 17 inches per second is the minimum linear speed necessary to insure flat frequency response to 20,000 cycles per second. Referring to TABLE I, it is seen that a linear speed of 17.00 inches per second occurs at a radius of 4.87 inches on a standard 33.33 R.P.M. record. Between this radius and the outer edge of the record the linear speed is higher--of no great detriment to response, but detrimental to the record surface in that excessive wear results. Below this radius the linear speed will decrease continuously to a minimum of 8.70 inches per second at a radius of 2.50 inches, which is about as close to the center of the record as is practical on conventional records. The decrease of sound quality and increase in distortion on the inner grooves of a record due to the lack of high frequency resolution is readily apparent upon playback with reasonably sophisticated equipment. It is this factor which prevents the phonograph disc as it currently exists from being a satisfactory recording medium wherein optimum sound quality is required.
To correct this intrinsic defect, the linear speed must be held constant at 17.00 inches per second. Although a linear speed lower than 17.00 inches per second would suffice for less critical applications, it is the intent and objective of this disclosure to extend the limits of recorded sound quality far beyond those of currently available commercially produced recordings. The above mentioned speed of 17.00 inches per second is a reasonable compromise between acceptable frequency response and sufficient playing time per side of the disc.
There has been to the present date no acceptable technique or apparatus for maintaining a record disc in a constant linear speed in relation to the stylus of the turntable of a playback apparatus. The achievement of such control will necessarily be implemented with electronic hardware since mechanical means would be inefficient, complicated and highly inaccurate. In addition, the apparatus and technique would have to be compatible with currently existing disc recordings, otherwise conventional recordings could not be played on a constant linear speed turntable.
Thus a requirement of a constant linear speed turntable is that it must be compatible with conventional techniques and apparatus. It must be able to reproduce conventional recordings at speeds of 78, 45, and 33.33 R.P.M. and it must be quickly and easily converted from constant R.P.M. (constant angular) mode to constant linear speed mode and vice-versa. The same electronic mechanism must be used to control both modes.
Another design requirement is that the constant linear speed turntable maintain a rumble level, i.e., the mechanical noise generated by turntable rotation; which is as low or lower than that currently existing in conventional turntables. In order to maintain low rumble, wow and flutter levels, moving parts in the turntable mechanism should be kept to a minimum.
Another design requirement is to provide a constant linear speed control mechanism which will be compatible with all tonearm configurations, including:
(1) standard pivoted arm; PA1 (2) pivoted arm with articulated (pantographic) pick-up head; and PA1 (3) straight-line radial tracking arm.
The constant linear speed control mechanism should cause no mechanical drag whatsoever on the tonearm, nor should it increase the moving mass of the tonearm.
A linear speed control mechanism should also be equally adaptable to a recording turntable as well as reproducing turntable.
A linear speed control technique, to be commercially acceptable, should additionally be independent of the rate of motion of the tonearm due to the inward spiraling of the record groove, as the groove spiral is irregular, and should also be independent of the position of the tonearm with respect to the disc. The linear speed correction must be fact enough that, should the tonearm be manually picked up and moved from one position to another on the disc, the necessary speed correction will take place before the stylus again touches the disc surface. The linear speed must be variable by several percent around the nominal value so that the speed can be accurately adjusted by the user by means of a stroboscope or a special closed-groove test record.